Study group XV of the CCITT (Consultative Committee on International Telephony and Telegraphy) which is an organization to standardize coding methods for video signals has recommended an interframe coder for low bit rate transmission of visual telephony signals, known as model H.261(CCITT, 1989). The CCITT H.261 standard has been developed for use in video teleconferencing and video telephony applications. It can operate at a frequency range from 64 kilobits(Kbits) per second to 1.92 megabits(Mbits) per second, and upon images between 525 and 625 lines in a common intermediate format(CIF).
Because of the severe demand to greatly reduce data for transmission, the low bit rate coding system such as the one recommended for use in a video telephone makes use of a spatial subsampling technique whereby only a limited number of pixels is sampled horizontally or vertically across an image before any coding thereof takes place. In view of the severe demand of data reduction, however, it would be desirable to develope a system capable of reducing the amount of data even before it is processed by the low bit rate coding system. One of such approaches as conceived by the present inventor is to select a given region or window having a particular feature, e.g., a facial picture, within an image captured by a video camera provided within the video telephone and then apply the coding process to the selected window only.